Biological molecules

Question 1

Monomers and polymers

Answer 1

A monomer is a small, single molecule, many of which can be joined together to form a polymer

A polymer is a large molecule made up of many similar or identical monomers joined together

Question 2

Monosaccharides and the resulting disaccharides

Answer 2

Monosaccharides are the monomers from which larger carbohydrates are made e.g. glucose, fructose and galactose

Glucose + glucose = maltose
Glucose + fructose = sucrose
Glucose + galactose = lactose

A condensation reaction between 2 monosaccharides forms a glycosidic bond

Question 3

Isomers of glucose: alpha and beta glucose

Answer 3

C6H12O6

Isomers have the same molecular formula but differently arranged atoms

Difference in structures between alpha and beta glucose is that the OH group is below C1 on a-glucose but above C1 in B-glucose

Question 4

Triglycerides what do they do

Answer 4

Triglycerides are energy-storage molecules

Question 5

Triglycerides how are they formed

Answer 5

Formed by the condensation of 1 molecule of glycerol and 3 fatty acids.

The condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond

Question 6

Triglycerides properties related to structure

Answer 6

They have a high ratio of C-H bonds to C atoms in the hydrocarbon tail so they release more energy than the same mass of carbohydrates.

They are insoluble in water (clump together as droplets) so no effect on the water potential of the cell.

Question 7

Phospholipids what are they

Answer 7

1 molecule of glycerol, 2 fatty acids, a phosphate-containing group.

Phosphate head, fatty acid tails

Question 8

Phospholipids properties related to structure

Answer 8

Phosphate heads are polar/hydrophilic so they are attracted to water. Orients to the aqueous environment either side of the membrane

Fatty acid tails are non-polar/hydrophobic so they are repelled by water. Orients to the interior of the membrane so that it repels polar/charged molecules.

Question 9

Phospholipids what do they do

Answer 9

Forms bilayer in the cell membrane, allowing diffusion of non-polar/small molecules

Question 10

Saturated fatty acids

Answer 10

No C=C bonds in hydrocarbon chain; all carbons fully saturated with hydrogen

Question 11

Unsaturated fatty acids

Answer 11

One or more C=C double bonds in hydrocarbon chain

Question 12

Emulsion test for lipids

Answer 12

1.) add ethanol and shake (to dissolve lipids)
2.) add water
3.) positive test: milky/cloudy white emulsion

Question 13

Condensation reaction

Answer 13

Joins 2 molecules together

Eliminates a water molecule

Forms a chemical bond

Question 14

Hydrolysis reaction

Answer 14

Separates 2 molecules

Requires addition of a water molecule

Breaks a chemical bond

Question 15

Glycogen structure and function

Answer 15

Energy store in animal cells

Polysaccharide of α-glucose with C1-C4 and C1-C6 glycosidic bonds, so it is branched

Question 16

Glycogen structure related to function

Answer 16

Branched; can be rapidly hydrolysed to release glucose for respiration to provide energy

Large polysaccharide molecule, cannot leave cell

Insoluble in water; water potential of cell is not affected, therefore there is no osmotic effect.

Polymer of glucose so easily hydrolysed

Glucose (polymer) so provides respiratory substrate for energy (release);

Question 17

Starch structure and function

Answer 17

Energy store in plant cells

Polysaccharide of α-glucose. Mixture of amylose and amylopectin.

Amylose has C1-C4 glycosidic bonds so it is unbranched, whereas amylopectin has C1-C4 and C1-C6 glycosidic bonds so it is branched

Question 18

Structure of starch related to its function (amylose)

Answer 18

Helical; compact for storage in the cell

Large polysaccharide molecule; cannot leave the cell

Insoluble in water, does not affect the water potential of the cell so there is no osmotic effect

Question 19

Cellulose function

Answer 19

Provides strength and structural support to plant cell walls

Question 20

Cellulose structure related to function

Answer 20

Every other beta-glucose molecule is inverted in a long, straight unbranched chain

Many hydrogen bonds link parallel strands to form microfibrils (strong fibres)

Hydrogen bonds are strong in high numbers, provide strength and structural support.

Question 21

Benedicts test for reducing sugars

Answer 21

Add Benedicts reagent (blue due to copper (ii) sulfate) to the sample

Heat in a water bath

Positive = red precipitate (copper (ii) sulfate reduced to copper (i) oxide)

Question 22

Benedicts test for non-reducing sugars

Answer 22

Add a few drops of dilute hydrochloric acid (hydrolyse sugar into its constituent reducing sugars)

Heat in a water bath

Neutralise with sodium bicarbonate

Add Benedicts solution and reheat

Positive = red precipitate

Question 23

How a bond forms between amino acids

Answer 23

A condensation reaction between 2 amino acids forms a peptide bond

Question 24

Protein primary structure

Answer 24

Sequence of amino acids in a polypeptide chain

Question 25

Protein secondary structure

Answer 25

Hydrogen bonding between amino acids (between carbonyl O of one and amino H of another)

Causes polypeptide chain to fold into repeating pattern eg alpha helix

Question 26

Protein tertiary structure

Answer 26

Overall 3D structure of a polypeptide held together by interactions between amino acid side chains:

Ionic bonds/disulfide bridges/hydrogen bonds

Question 27

Quaternary structure of proteins

Answer 27

Some proteins are made of 2+ polypeptide chains

Held together by more hydrogen, ionic and disulfide bonds

eg haemoglobin

Question 28

Test for proteins

Answer 28

Add Biurets solution: sodium hydroxide + copper (ii) sulfate

Positive = purple

Detects presence of peptide bonds

Question 29

Enzymes what do they do

Answer 29

Lowers the activation energy of the reaction it catalyses -> speeds up rate of reaction

Question 30

Lock and key model

Answer 30

Old, outdated

Active site is a fixed shape, it is complementary to one substrate

After a successful collision, an enzyme-substrate complex forms leading to a reaction

Question 31

Induced fit model

Answer 31

Recent, accepted

Before reaction, active site is not completely complementary to the substrate

Active site shape changes as substrate binds and enzyme-substrate complex forms.

This stresses/distorts bonds in substrate leading to a reaction

Question 32

Specificity of enzymes

Answer 32

Enzymes have a specific shaped tertiary structure and active site.

Active site is complementary to a specific substrate

Only this substrate can bind to the active site, inducing fit and forming an enzyme-substrate complex.

Question 33

Concentration effect on enzyme-controlled reactions

Answer 33

Increasing concentration of enzymes/substrate -> rate of reaction increases.

More enzymes/substrates, more available active sites, more successful enzyme-substrate collisions.

Plateaus due to enzyme/substrate limiting factor.

Question 34

Temperature effect on enzyme-controlled reactions

Answer 34

Increasing temperature up to optimum -> rate of reaction increases

Increase in kinetic energy, more successful enzyme-substrate complexes.

Increase temperature above optimum: rate of reaction decreases as enzymes are denatured - their tertiary structure and active sites change shape and hydrogen/ionic bonds break.

Question 35

pH effect on enzyme-controlled reactions

Answer 35

pH above/below optimum pH -> rate of reaction decreases

Enzymes are denatured - their tertiary structure and active sites change shape and hydrogen/ionic bonds break.

Complementary substrate can no longer bind to active site

Fewer collisions

Question 36

Competitive inhibitors

Answer 36

Decrease rate of reaction

Similar shape to substrate so competes for active site so substrates can’t bind.

Fewer enzyme/substrate complexes

Increasing substrate concentration reduces effect of inhibitor

Question 37

Non-competitive inhibitors

Answer 37

Decrease rate of reaction

Binds to site away from the active site so that the enzymes tertiary structure changes shape so substrate cannot bind to the active site anymore

Fewer enzyme/substrate complexes

Increasing substrate concentration has no effect on the rate of reaction as there is permanent change on the active site.